1. Technical Field of the Invention
The present invention relates generally to a pulsed phase locked loop and more particularly to improvements in a pulsed phase locked loop to measure small changes in the ultrasonic path length of a material and accordingly calculate stress in the material.
2. Discussion of the Related Art
A pulsed phase locked loop (P2L2) strain monitor is described in U.S. Pat. No. 4,363,242 to Heyman and is used to nondestructively measure the load on a bearing member such as a bolt, connector, etc. The components and operation of this P2L2 monitor are discussed in greater detail below. A problem of this monitor is that a low pass filter located after a mixer phase detector causes errors in the phase measurement due to the step response of the low pass filter. Although this induced error is not significant enough to prevent the P2L2 monitor from tracking small changes in frequency, it does corrupt the spacing of the lock points in such a way that transducer calibration is not possible since the calibration requires reacquisition of the same lock point for each transducer. The recertification of a loaded member described in the related applications depends on the reacquisition of certain specified lock points. In addition, use of a voltage controlled oscillation (VCO) slows the speed of data acquisition, prevents the system from being automated, and increases the complexity of the monitor by requiring a very accurate, e.g., within 1 Hz frequency measurement.